But it should not necessarily please us. The ideas at the heart of Greene's book are both unproven (as he cheerfully admits) and corrosive. They violate what I call "The Moral Principle." Not to worry, I will explain. But first, some context.

Big Ideas, such as Newton's universal gravity, Darwin's descent with change from a common ancestor, and Einstein's relativity, are tremendous achievements in science. More than monuments in intellectual history, they make everyday life better. Newton laid the framework for flight; Darwin's theory makes new cures possible; and Einstein's relativity enables GPS, to name a few "apps."

All the same, Big Ideas have a way of filtering into popular culture, where they sometimes mutate and do harm. The prime example is eugenics, a mishmash of Darwin's theory of evolution and Herbert Spencer's political philosophy.

Invented by Francis Galton, a cousin of Darwin's, this pseudoscience caught the imagination of many powerful nonscientists, ranging from Theodore Roosevelt to Adolph Hitler. Unfortunately, it is all too easy for people in power to imagine that they know bad genes when they see them. The harm that followed was appalling. Forced sterilization, institutionalization of "morons," and, some say, the Holocaust.

Opponents of evolution like to argue that Darwin's theory was responsible for all these ills. This is demonstrable nonsense, but even if the charge were valid I would resolutely defend the teaching and application of evolutionary theory for one simple reason: all the evidence supports it. Evolution, as an explanation for the diversity and common ancestry of life, towers above other major scientific theories as the best supported, most coherent and most fruitful of them all.

Contrast that with the conjecture of parallel universes. It grows out of quantum physics. That's a problem to begin with: Quantum theory, though experimentally verified to an astonishing number of decimal places, is incomplete. Something in our understanding is wrong, because it doesn't fit with the also verified theory of general relativity. Quantum and gravity theories mix like oil and politics.

Greene and many others believe that string theory may resolve that conflict. Indeed, it might. But string theory exists entirely in dense mathematics. None of it can so far be tested, and much of it lies beyond confirmation in principle. Yet, from these mathematical speculations arise conjectures of parallel universes set in an infinite multiverse. It's quite a leap.

Let's get specific. There are numerous varieties of parallel universe conjectures. The late Hugh Everett III created the Many Worlds theory to explain how quantum "choices," such as whether a beam light will be a wave or a packet of particles, are made. He argued that at every decision point, the universe splits in two, with the decision being realized in opposite ways in each new whole. Possible, but profoundly absurd.

More recently, the mathmanauts who explore string theory have found hints that the gigantic bubble we inhabit may be just one of an infinite number of such bubbles, each a random dice-roll of particles and laws. That's where the trouble starts. For, if true, it means that every possible variation exists, and not only exists, but exists an infinite number of times. So, as in Star Trek, there are worlds where the Roman Empire never fell, worlds where Hitler won and so on. But there are also worlds that are precise duplicates of ours -- indeed, an infinite number of such worlds.

Though there be method in it, that way madness lies.

Enter, the Moral Principle. It states that we should resist accepting any proposition that tends to disable moral reasoning, unless and until the scientifically interpreted evidence compels us.

I honed this principle in the context of my critique of religion, but it applies, for example, to the secular idea of the philosopher's zombie. The Moral Principle prevents us from accepting the idea that anyone else is a zombie who appears to be just like any other person, except that there's no real consciousness inside. If we were to accept that idea, there would be no moral barrier to torturing or murdering "zombies." In fact, it would be much like Hitler's dehumanization of the Jews.

Now, please don't think that I am accusing Brian Greene of any such intent, let alone any misdeed. He seems to be a likable man, and he's clearly brilliant. His ideas are sound, and he is careful to qualify his claims.

The danger lies in how they take root in popular culture. If we come to believe that choices do not matter, that any action is matched by its opposite somewhere, we risk losing our capacity for moral reasoning. History shows that, inbuilt though that capacity may be, ideas can short-circuit it.

In short, what I am saying is that those of us who are NOT so brilliant as to be able to follow the math need to resist being seduced by visions of parallel bubbles in a multiverse. They may exist. Indeed, I think it likely that they do.

Whether they exist in untrammeled profusion, however, is another question. It is one that has to be weighed against the possibility that, orderly as our universe generally is, there may be a higher order to the arrangement of the multiverse. In fine, lacking definitive evidence we need to keep alive the possibility of a moral order. If there is indeed a multiverse, it may yet prove to be but a part of the ultiverse.